It is known that an internal combustion engine, for example a Diesel engine, includes at least one fuel injector provided for injecting metered quantity of fuel into a corresponding engine cylinder. The fuel injector is controlled by an electronic control module (ECM) which is generally configured to determine a fuel requested quantity that should be injected into the engine cylinder and to command the fuel injector to inject said fuel requested quantity. However, production spread and tolerances have the effect that the quantity of fuel actually injected by the fuel injector is generally different from the fuel requested quantity and this difference may have a relevant impact especially when a small fuel quantity is concerned (e.g. a pilot injection), thereby increasing pollutant emissions, combustion noises and vibrations.
In order to guarantee that the fuel injector is able to actually inject small fuel quantities that correspond to the requested fuel quantities with sufficient accuracy, a learning procedure is usually carried out at the end of the production line of the internal combustion engine and/or after any replacement of the fuel injector. This learning procedure generally provides for the ECM to adjust the fuel requested quantity by controlling the engine speed (i.e. the rotational speed of the internal combustion engine) in a closed-loop control, so that the engine speed matches a target value thereof (e.g. the idle speed).
While the internal combustion engine is operating in this way, the ECM calculates a reference value of the fuel requested quantity that corresponds to the fuel quantity that would be requested from a nominal fuel injector in order to bring and keep the engine speed at the target value. This reference value is subtracted from the value of the fuel request quantity under which the fuel injector is actually commanded by the closed-loop control and such difference is memorized to correct the values of the fuel requested quantity during the normal operation of the internal combustion engine.
The calculation of the aforementioned reference value of the fuel requested quantity is conventionally carried out on the basis of a PWM signal, usually referred as to F-terminal signal, which is generated by an electric generator (e.g. an alternator) coupled to the internal combustion engine and whose duty-cycle represents an amount of mechanical energy that the internal combustion engine is supplying to said electric generator in order to charge an electric battery. However, the F-terminal signal does not comply with the severe diagnostic protocols required by the OBD-II standards, so that the entire learning strategy is not OBD-II compliant.